N-phosphonomethylglycine, otherwise known as glyphosate, is well known in the art as an effective post-emergent foliar applied herbicide. Glyphosate is an organic compound with three acidic groups, and in its acid form is relatively insoluble in water. Glyphosate is, therefore, normally formulated and applied as a water-soluble salt. Although monobasic, dibasic and tribasic salts of glyphosate can be made, it has generally been preferred to formulate and apply glyphosate, in the form of a monobasic salt, for example as a mono-(organic ammonium) salt such as the mono(isopropylamine), often abbreviated to IPA, salt. Roundup.RTM. herbicide containing the IPA salt of glyphosate is sold by Monsanto Company as an aqueous solution concentrate (SC) formulation which is generally diluted in water by the user prior to application to plant foliage. The mono-(inorganic ammonium), i.e., NH.sub.4.sup.+, salt of glyphosate is also commercially available, mainly contained in dry water-soluble granule (SG) formulations such as Rival.RTM. herbicide and Roundup.RTM. Dry herbicide of Monsanto Company, which are dissolved in water by the user prior to application to plant foliage. Other commercial glyphosate products include Touchdown.RTM. herbicide of Zeneca, an aqueous SC formulation containing the mono(trimethylsulfonium), often abbreviated to TMS, salt of glyphosate, and Roundup.RTM. Geoforce herbicide of Monsanto Company, a dry SG formulation containing the monosodium salt of glyphosate.
When the terms "ammonium", "monoammonium" and "diammonium" are used herein to refer to salts of glyphosate, these terms apply strictly to inorganic ammonium, i.e., NH.sub.4.sup.+, unless the context demands otherwise. Glyphosate rates and concentrations given herein, even where the glyphosate is present as a salt or salts, are expressed as acid equivalent (a.e.) unless the context demands otherwise.
Various salts of glyphosate, methods for preparing salts of glyphosate, formulations of glyphosate or its salts and methods of use of glyphosate or its salts for killing and controlling weeds and other plants are disclosed in U.S. Pat. No. 4,507,250 to Bakel, U.S. Pat. No. 4,481,026 to Prisbylla, U.S. Pat. No. 4,405,531 to Franz, U.S. Pat. No. 4,315,765 to Large, U.S. Pat. No. 4,140,513 to Prill, U.S. Pat. No. 3,977,860 to Franz, U.S. Pat. No. 3,853,530 to Franz, and U.S. Pat. No. 3,799,758 to Franz. The aforementioned patents are incorporated herein in their entirety by reference.
Glyphosate salts generally require the presence of a suitable surfactant for best herbicidal performance. The surfactant may be provided in the concentrate formulation, or it may be added by the end user to the diluted spray solution. The choice of surfactant is very important. For example, in an extensive study reported in Weed Science, 1977, volume 25, pages 275-287, Wyrill and Burnside found wide variation among surfactants in their ability to enhance the herbicidal efficacy of glyphosate.
Use of a highly concentrated aqueous formulation of glyphosate in the form of a salt made with the inorganic base ammonia would be advantageous. Ammonia is lower in cost than most other bases, readily available, low in molecular weight, relatively highly soluble in water and a natural nutrient for the growth of plants and other organisms. The use of ammonium salts of glyphosate for preparing aqueous concentrate formulations of glyphosate suitable for killing and controlling weeds and other plants has, however, been limited until the present invention disclosed herein. The limitations reported have included one or more of the following: difficulties arising from chemical and physical properties of the ammonium salts of glyphosate, lack of definition of suitable surfactants for preparing high-loaded liquid concentrates of such salts, reduced weed control, and requirement for complex processes for preparing liquid ammonium glyphosate compositions substantially lower in the amount of glyphosate acid equivalent than the compositions of the present invention.
Preparations of aqueous herbicidal compositions of glyphosate, all or part of which is in the form of its monoammonium and/or diammonium salts, have been disclosed. European Patent No. 0 290 416 discloses aqueous concentrates containing a cationic polyoxyalkylene alkylamine (including alkenylamine) surfactant together with glyphosate, a portion of which is in the form of the monoammonium salt and another portion of which is in the form of a salt wherein the surfactant provides the counterion. The formulations composed partly of monoammonium glyphosate as disclosed in this European Patent have a maximum total concentration of glyphosate in all forms present of 300 grams of acid equivalent per liter (g a.e./l) and are formed by a multi-step process involving separately preparing the monoammonium and surfactant salts of glyphosate in order to be able to combine them in mixture.
In Weed Research, 1996, volume 36, pages 241-247, Nalewaja, DeVilliers and Matysiak disclose the use as herbicides of compositions containing monoammonium and diammonium glyphosate as well as compositions based on the isopropylammonium, sodium and calcium salts of glyphosate. These compositions did not contain any surfactants as prepared. Two nonionic surfactants and one cationic surfactant were separately added, immediately before application to plant foliage, to relatively dilute solutions with indicated maximum glyphosate concentration of less than 10 g a.e./l. The monoammonium and diammonium salt compositions reportedly exhibited less herbicidal activity and glyphosate uptake than the corresponding isopropylammonium salt compositions and required the addition of surfactant separately.
Herbicidal aqueous concentrate formulations of water-soluble salts of glyphosate containing ammonium sulfate have been reported in European Patent 0 274 369. The glyphosate used was reported to be in the form of the isopropylammonium salt; however the presence of ammonium cations together with glyphosate anions resulting from introduction of ammonium sulfate could be considered to provide ammonium glyphosate.
Water-soluble dry solid compositions based on monoammonium glyphosate as agriculturally acceptable herbicidal formulations containing effective surfactants have been developed to overcome some of the aforementioned limitations. As disclosed in European Patent Application No. 0 378 985 and European Patent Application No. 0 582 985, the usefulness of these solid non-aqueous formulations has been limited by one or more of the following: the generally complex, lengthy, and relatively expensive processes for preparation, inconvenient handling properties, and difficulties in agronomic usage. In addition, although many farmers and agricultural spray operators exhibit no strong preference between dry and liquid formulations, a substantial proportion strongly prefer to use liquid, especially aqueous, formulations for a variety of reasons.
It is, therefore, an objective of the present invention to provide a storage-stable, highly concentrated aqueous formulation containing an ammonium salt of glyphosate in a herbicidally effective amount together with a herbicidal efficacy enhancing amount of one or more agriculturally acceptable surfactants.
This objective is rendered particularly difficult by the problems of surfactant compatibility with ammonium glyphosate solutions of high ionic strength. For example, in a direct comparison of mono(isopropylammonium) glyphosate and monoammonium glyphosate, it has been found that, at equal glyphosate a.e. concentration in water, a monoammonium glyphosate solution is significantly more difficult to formulate as a storage-stable concentrate than a mono(isopropylammonium) glyphosate solution. This increased difficulty is manifested in various ways, as outlined below.
The range of surfactants compatible with monoammonium glyphosate at, for example, a glyphosate concentration of 360 g a.e./l and a surfactant concentration of 180 g/l, is much more restricted than the range of surfactants compatible with mono(isopropylammonium) glyphosate at the same glyphosate and surfactant concentrations. As one instance, at the illustrative concentrations given immediately above, the surfactant MON 0818 of Monsanto Company, based on polyoxyethylene (15) tallowamine, shows excellent compatibility with mono(isopropylammonium) glyphosate, but is incompatible with monoammonium glyphosate, this incompatibility resulting in immediate phase separation, or "salting out" of the surfactant from the glyphosate salt solution. PA1 For a given surfactant at a given concentration, the maximum concentration, or "loading", of glyphosate a.e. in a storage-stable aqueous concentrate formulation is, for most surfactants, significantly lower in the case of the monoammonium salt than in the case of the mono(isopropylammonium) salt of glyphosate. PA1 For a given loading of glyphosate a.e., the maximum concentration of most surfactants that can be accommodated in a storage-stable aqueous concentrate formulation is significantly lower when the glyphosate is present as the monoammonium salt than as the mono(isopropylammonium) salt. PA1 Additional compatibilizing agents such as octylamine hydrochloride that are unnecessary in aqueous concentrate formulations of mono(isopropylammonium) glyphosate with a particular surfactant can be required for acceptable storage stability in the case of monoammonium glyphosate. PA1 At given glyphosate a.e. and surfactant concentrations, the maximum temperature that an aqueous concentrate formulation can withstand without phase separation is generally lower with monoammonium glyphosate than with mono(isopropylammonium) glyphosate. An indication of this maximum temperature is obtainable by measuring cloud point by methods known to those of skill in the art. PA1 1. quaternized long chain amines: PA1 2. quaternized polyoxyalkylene long chain amines: PA1 3. quaternized pyridines: PA1 4. quaternized carboxylated imidazolines (closed and open chain): PA1 5. trialkylbetaines: PA1 6. amine oxides:
It is a further objective of the present invention to provide a storage-stable, highly concentrated aqueous formulation containing an ammonium salt of glyphosate and one or more surfactants and having a superior level of herbicidal effectiveness by comparison with current commercial standard glyphosate salt formulations such as Roundup.RTM. herbicide or Touchdown.RTM. herbicide.
The difficulty of meeting this objective is compounded by the fact that herbicidal effectiveness of glyphosate salt solutions is highly dependent upon two factors: selecting a suitable surfactant and providing as high a concentration of that surfactant as possible in the concentrate formulation. Using the monoammonium salt of glyphosate in place, for example, of the mono(isopropylammonium) salt militates against both of these factors.
Even without replacing the salt of glyphosate, it is difficult to enhance the herbicidal effectiveness of the present Roundup.RTM. and Touchdown.RTM. formulations, containing respectively the mono(isopropylammonium) and mono(trimethylsulfonium) salts. Any substantial increase in the concentration of surfactant in these products is achievable only at the expense of reducing glyphosate a.e. loading. Likewise, any substantial increase in glyphosate a.e. loading of these products is achievable only at the expense of surfactant concentration and therefore brings with it at least a risk of reduced herbicidal effectiveness.
Thus the hurdle to be surmounted in developing an aqueous ammonium glyphosate formulation that meets all the criteria of (i) having a high glyphosate a.e. loading, (ii) containing a suitable herbicidal efficacy enhancing surfactant, and (iii) having a high enough concentration of that surfactant to provide herbicidal effectiveness greater than that of commercial standard glyphosate salt formulations, is truly a high one. Nonetheless, the objectives set out above have now been met by the invention described and claimed herein.